(i) Technical Field
The present invention relates to a compound semiconductor device.
(ii) Related Art
There is a demand for higher voltage and higher electrical power density of a semiconductor device. A wide band-gap semiconductor such as a nitride semiconductor is expected as a material meeting the demand. In particular, a nitride semiconductor such as GaN is expected, because GaN has wide band-gap, high thermal conductivity, hetero junction characteristics and so on. A HEMT structure having a channel layer made of AlGaN on an electron transit layer made of GaN is known as a transistor using a nitride semiconductor such as GaN.
However, there is a problem that current collapse should be restrained in the transistor having the nitride semiconductor. The current collapse is a phenomenon that current output is reduced because of large output operation at high frequency. And so, Japanese Patent Application Publication No. 2005-286135 discloses a case where a GaN thin layer (hereinafter referred to as a GaN cap layer) is provided on a channel layer made of AlGaN for a purpose of restraining the current collapse.
However, the current collapse should be restrained more in order to make a device of higher frequency and higher outputting. And so, the present inventors have researched a position of a field plate electrode between a gate electrode and a drain electrode in a transistor. However, parasitic capacity may occur with respect to the drain electrode because the field plate electrode is at a source potential (ground potential). The parasitic capacity is inescapable in high-frequency operation.